Vacuum interrupters disposed typically in vacuum circuit breakers and vacuum switchgears each have a pair of electrical contacts capable of being turned on and off. Receiving and distributing equipment such as vacuum circuit breakers must be downsized. To reduce such vacuum interrupters in diameter and size, the interruption performance of electrical contacts of the vacuum interrupters must be improved so as to interrupt a heavy current at electrical contacts with a small area. Chromium-copper (Cr—Cu) electrical contacts are predominantly used as electrical contacts having excellent interruption performance (Patent Document 1).
If current of the vacuum interrupter used in an inductive circuit is interrupted, abnormal surge voltage is induced, which may lead to insulation breakage of electrical equipment. The chopping current must be reduced so as to suppress the abnormal surge voltage. Accordingly, another one of requirements for electrical contacts is a small chopping current. As electrical contacts having small chopping current and low surge voltage, Co—Ag—Se alloy electrical contacts have been known (Patent Document 2 and Patent Document 3).
[Patent Document 1] Japanese Unexamined Patent Application Publication (JP-A) No. 2005-135778
[Patent Document 2] Japanese Unexamined Patent Application Publication (JP-A) No. Hei 07-029461
[Patent Document 3] Japanese Unexamined Patent Application Publication (JP-A) No. Hei 09-171746
The vacuum circuit breakers typically using Cr—Cu alloy electrical contacts are excellent in interruption performance and can interrupt a large current, but cause a surge voltage upon interruption of large current. Accordingly, they must use a surge absorber for absorbing the abnormal surge voltage, and this leads to increase in size and cost of electrical equipment.
The vacuum circuit breakers typically using Co—Ag—Se alloy electrical contacts show a low surge voltage but are unsuitable for large-current interruption.
The interruption performance and the low-surge property are considered to be theoretically incompatible with each other, because the current is interrupted at a higher value than zero to yield a larger chopping current with an increasing interruption performance. Accordingly, electrical contacts having high interruption performance and those showing a satisfactorily low surge voltage are used case-by-case to suit the type and use of vacuum circuit breakers.
In addition, the vacuum circuit breakers must maintain required properties even after carrying out interruption many times, but electrical contacts combining excellent large-current interruption performance and low-surge performance may have reduced low-surge performance after carrying out interruption many times.